Ferromagnetic Materials
AimsBefore you startIntroductionTypes of magnetismWhy are some materials magnetic?Why are only some materials ferromagnetic?Curie-Weiss lawDomainsDomain wallsHysteresisAnisotropySummaryQuestionsGoing furtherTLP creditsTLP contentsShow all contentViewing and downloading resourcesAbout the TLPsTerms of useFeedbackCredits Print this page
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Summary
This TLP has covered the basic points of ferromagnetism:
- In a magnetic atom there are two contributions to the magnetic dipole moment; firstly the spin of the electrons themselves and secondly that of electrons orbiting the nucleus.
- Ferromagnetism occurs in materials where all the magnetic dipole moments align parallel below the Curie temperature.
- Ferromagnetic ordering was explained by Weiss via a hypothetical average field which acts to cause the parallel alignment. However, the microscopic explanation for this can be found by looking at the Pauli Exclusion Principle; it is energetically more favourable for electrons be placed in different orbitals as this reduces the Coulomb repulsion energy and allows for the alignment of the electron spins.
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The magnitude of the magnetisation is dependent on temperature and modelled by the Curie-Weiss law:
\[\chi = \frac{C}{{T - {T_c}}} = \frac{M}{H}\;\,\,\,{\rm{equation}}\;2\;\rm{in\;text}\]
- The formation of domains is driven by the minimisation of energy, with the main driving force often being that of the magnetostatic energy.
- Magnetic hysteresis is seen, due to the defects found in crystals, as these hinder the movement of domain walls.
- Hard magnets have a large coercive field, whereas soft magnets are easily demagnetised and so has a small coercive field.
- Both magnetocrystalline anisotropy and shape anisotropy give directions
in a material along which it is easier to magnetise a sample.